Digital fabrication is praised as the future of manufacturing. Computer Numerical Controlled (CNC) machine tools can convert a digital design into an object with the click of a mouse, which means the production process is completely automated. The use of digital machine tools has spread rapidly in factories over the last two decades, and they have now become cheap and user-friendly enough to bring them within reach of workshops and makers.

While CNC machines have been embraced by many, including some environmentalists who say the technology can be more sustainable, it's important to consider the very high energy use of digital machine tools. Compared to the earlier generation of human-controlled machine tools, CNC machines use much more power, and the potential to improve their energy efficiency is very limited. Choosing fewer automated technologies is the key to sustainable manufacturing.

Manufacturing processes can be broadly divided into two groups: primary and secondary. The former -- such as forging and casting -- provide basic shape and size to the material. Secondary processes provide the final shape and size with tighter control on dimension and surface characteristics. This second group is performed by so called "machine tools".

Machine tools form the basis of precision manufacturing. As such they are indispensable for the production of nearly all modern technology, be it cars, planes, household appliances, computers, solar panels, wind turbines, or other machine tools. They are so fundamental to manufacturing that the United Nations Industrial Development Organization (UNIDO) defines "industrialisation" as the capacity to produce machine tools. [1]

Machine tools are stationary operating machines that perform the geometric shaping of workpieces. [2] They are mostly subtractive technologies, dealing with any process in which material is removed gradually from a workpiece. There is a great diversity in specialized machine tools, but common examples are sawing and cutting machines, drilling machines, grinding machines, turning machines (lathes) and milling machines.

Machine tools can process different materials, but today most of them are used for the shaping of metal parts. Milling machines have become the most common metal shaping tools in the mechanical manufacturing industry. Milling is a process of cutting away material by feeding a workpiece past a rotating cutting tool. A milling machine operates like a human sculptor. It can have grinding, cutting and polishing heads.

A milling machine at work. Most milling machines have three axes, with a table that moves along the x-axis which sits on a saddle that moves along the y-axis. The spindle that holds the cutting tool is housed on a milling head on the z-axis. Picture: Wikipedia Commons.

Lathes or turning machines are similar to milling machines but they are used to shape cylindrical workpieces. In a milling machine the workpiece remains (relatively) stationary while material is removed by moving tools heads. In a lathe, the workpiece is rotating while the tool head remains (relatively) stationary.

How to Power a Machine Tool

Modern definitions of machine tools exclude any machine that is powered by humans. However, the predecessors of most machine tools were hand- or foot-powered. Woodworking hand tools with the same kinematic motions as modern machines, such as the cord lathe and the bow drill, were developed in ancient times. [3]

More sophisticated, foot-driven wood lathes, which had many of the essential features of modern turning machines, appeared in the middle ages. [4] Lathes for metalworking, which were originally developed for the production of helical threads, emerged in the 1700s. [5] The first real milling machine for metalworking was built in the early 1800s, although similar operations were performed on adapted lathes by clockmakers at least one century earlier. [5][6][3][4]

Machine tools were also driven by animals, wind or water power. The Romans already had water-powered sawmills for cutting marble, but most automatic machines only appeared in the middle ages. The Europeans used sawmills, boring mills, turning mills, grinding and sharpening mills, polishing mills, and metal slitting and rolling mills that were powered by water, wind or animals. [7] [8]

A Swedish water-powered metal cutting mill from the 18th century. Metal cutting mills had pairs of revolving cylinders through which metal was passed either to form sheets (rolling) or to be cut into strips or rods (slitting)

These industrial mills were stationary, complex machines using non-human power sources, and thus fit the modern definition of machine tools. However, from the 1850s onwards, machine tools were increasingly powered by steam engines, and today almost all machine tools are powered by electricity.

How to Operate a Machine Tool

The evolution of machine tools continued throughout the twentieth century, though most improvements were now made to the operation of the process. During the first half of the century, all machine tools were hand-operated. The cutting tool itself was powered by steam engines or electricity, but workers shaped the workpiece and took care of all auxiliary actions.

For example, in a milling machine, motors only ran the drill and mechanized the x-axis bench movement. [9] All other movements were initiated and controlled by the operator, one at a time, often by use of hand wheels. [10]

A manually-operated milling machine, 1920

These so-called "manual" machine tools are still in use, but they're becoming increasingly rare. In the early 1950s, the first automatic machine tool was developed. Data was fed into the machine on punched tape, which allowed the machine tool to work independently. These "numerically controlled" (NC) machine tools became commercially available in 1960. [4] Around 1980, tape control was replaced by computer control, which led to the development of "computer numerically controlled" (CNC) machine tools.

CNC machine tools heralded the era of digital manufacturing. With CNC machines, human input is limited to the creation of a digital design on a computer, which is then automatically converted into a physical object by the machine tool. To achieve this, the digital design is fed into software that converts it into a set of symbolically coded instructions arranged in a proper sequence. The computer of the CNC machine then executes the program step by step by moving the machines slides and the tool head accordingly. [4][11]

Increasing Automation

Most machine tools built after 1985 are CNC machines, but considering the long life expectancy of machine tools means the switch to CNC machines is still on-going.[3] For instance, in 1995, only 350,000 of the 3.5 million metal-working machine tools in Europe were CNC machines. In 2009, this number had increased to 750,000. [12] At the same time, the unit value of CNC machine tools raised significantly from 1995 to 2009, indicating that the machines have become more complex. [12]

The level of automation in a CNC machine can vary considerably. For example, in a milling machine, apart from the positioning and powering of the tool, auxiliary functions such as machine lubrication, tool changes, chip handling, tool break detection, or the loading and unloading of workpieces can all be set in automatic code, but they can also be operated by hand. [4][11] The trend, however, is a move towards more automated auxiliary functions. [13]

Automation also makes it possible to design more complex machine tools. There are now milling machines with four, five or more axes which also support rotation around one or multiple axes, while 3-axis milling machines can only cut away material horizontally and vertically.

Power Use Increases with Automation Level

It's obvious that the switch from human- and water-powered tools to fossil-fuel powered tools has made manufacturing less sustainable. What's surprising, however, is that the switch from human-controlled machine tools to computer-controlled machine tools has much larger consequences for energy use. Automation is more energy-intensive than mechanisation.

A comparison of the maximum power requirements by three CNC milling machines (from 1988, 1998 and 2000) and one hand-operated milling machine (from 1985), all cutting a similar workpiece, revealed that the digital machines require 2.5 to 67 [sic] times more power than the manual machine. At full operation, the hand-operated machine tool used 2.8 kW, while the digital machines used 7 kW for the 1998 machine, 9.4 kW for the 1988 machine, and 188 kW for the 2000 machine. [13][9]

The authors of this widely cited paper, Jeffrey Dahmus and Timothy Gutowski, show that the large difference in power use between several types of CNC milling machines is mainly due to different automation levels. The 2000 machine, which operates in a car factory, is fully automated, while the other two digital machines only have their basic milling operations controlled by computers.

Load-Independent Power Use

The power use of a machine tool is distributed among various activities. Constant start-up operations refer to the load-independent power use of the machine tool. This power is required whenever the machine is turned on, independent of whether or not a part is produced. Run-time operations include the power required for positioning the workpiece and loading the tools. Material removal operations refer to the actual power involved in cutting. [13]

In the case of the most automated CNC milling machine, the power used handling the workpiece and tools is less than 15% of the total power required. The remaining 85% of the power used by the machine is constant, even when no action takes place. [13] For the other machines, this share is lower, but still significant. In absolute figures, power use for constant start-up operations (= idling) is 166 kW for the most automated milling machine, 1.3 kW to 3.4 kW for the less automated milling machines, and 0.7 kW for the manual machine. [13] [14]

In all four of the milling machines analysed by Dahmus and Gutowski, the power required to actually cut the material is the same, assuming operating parameters, materials properties, and tool characteristics remain constant. [13] If the material removal rate would be the same for each machine, milling a similar part on a manual machine would take 2 to 5 times less energy than on the least automated CNC machine tools, and 240 times less energy than on the most automated CNC milling machine.

Material Removal Rate

Operating parameters are not the same, however. The material removal rate is 3 to 13 times higher on the more automated machines. [15] Because energy consumption equals power consumption multiplied by time, in the end a CNC machine might use less energy than a manual milling machine for the processing of a similar part.

Higher production rates also increase the power consumption at the tool tip: from 2 kW for the manual machine, to 6 kW for the least automated CNC machines and to 22 kW on the fully automated CNC machine. [13] Higher production rates require stiff mechanical systems with the capability to absorb arising inertia forces. As a consequence, the masses of the machine structure, such as moving machine components, have to be increased. This, in turn, require motors with high torque output which are able to increase the forces needed during acceleration and deceleration. [16]

On the other hand, because of a higher production rate, the high power required for maintaining the CNC machine tools in a "ready" position can be divided over a larger amount of parts. If you calculate the energy used per unit of material removed, then the two less automated CNC milling machines have a similar efficiency as the manual machine. Assuming 1,000 work hours (30% of machine time positioning, 70% of machine time cutting, and 0% idling), the human controlled machine requires 6.2 kJ of energy to remove one cubic centimetre of material, while the CNC milling machines require 4.8 to 7.1 kJ of energy. [13]

The fully automated milling machine, on the other hand, requires 39 kJ per cubic centimetre of material removed, and is thus at least five times less efficient per unit of material removed than the other machines. [13]

Idling Time

An idling time of 0%, as we have assumed, is not realistic. In fact, machine tools are rarely switched off between jobs and often remain idling even at night, notes the German Fraunhofer Research Institute in a 2012 study on sustainable manufacturing methods:

"There are various reasons why machine tools are frequently not shut off at the end of a shift: One reason is to ensure thermal steady-state conditions with a specific thermal strain, which is very important for high accuracy of metal working machine tools, complex products and work pieces. Overnight shut off or other periods of non-operation of machine tools could result in processing temperatures to fall below desirable levels, and thus having an adverse impact on the process and manufacturing precision." [17]

When idling times increase, the energy use per cubic centimetre of material removed increases significantly for digital machines, and much less so for the hand-operated machine tool. If the two least automated milling machines are kept idling all night, their specific energy consumption would be much larger than that of the manual machine. This would be even more so for the fully automated CNC machine, but Dahmus and Gutowski note that these types of machines are often operated 24 hours per day and are idling less than 10% of their total life time. [13]

In theory, there are many opportunities to improve the energy efficiency of machine tools. For instance, engineers have designed alternatives to coolant and lubrication systems, as well as kinetic energy recovery systems. Further energy savings could be achieved by using lightweight but strong materials, more efficient motors, more flexible machines, a transition to power saving modes during stand-by periods, and so on. [18][16][19]

However, the energy savings potential of these technologies seems rather small. According to the Fraunhofer Institute, "there is no single option with a large environmental improvement potential, and moderate savings of 3-5% can be realised only with the implementation of several individual options". For metal working machine tools, they estimate the savings potential at 4%." [20]

The problem is not just technological. The main obstacle is the extent to which machine tool builders are willing to integrate energy-saving technologies. Few manufacturers have used the best available technologies. [2] Kinetic recovery systems, for instance, although considered a big trend in the 1990s, were never broadly introduced into the market. [16]

Sustainability versus Productivity

Incentives to improve the energy efficiency of machine tools are rather small. Firstly, energy saving measures may impede productivity. For instance, when taking into account the large power use during idling, it's obvious that a power saving mode during stand-by periods could be as useful on a machine tool as it is on a laptop. However, the European Association of the Machine Tool Industries notes that:

"Productivity is severely hampered, if too short transition periods from any processing mode to a sleep/standby-mode of the machine tool or parts thereof are made obligatory: warm up and bringing back the machine to full operation state delays the processing". [21]

Secondly, depending on the automation level, energy costs are only 1 to 6% of total machining costs. [22][17] A study investigating energy and cost-efficiency in CNC machining concludes that "the energy prices of today are not high enough to pose any particular need for making radical energy savings for CNC machining". Instead, the researchers advice that "significant cost savings can be achieved if the production output is increased as a consequence from higher removal rates.... If production output can be increased, cost and energy savings can follow". [22]

Larger cost savings can be achieved by increasing the production rate rather than by using more energy efficient machine tools, so there is little to win by concentrating on energy use -- especially when energy saving measures are in conflict with higher production rates.

Increasing the Production Rate

Further increasing the production rate of digital machining tools will save money, but energy savings are less certain. As we have seen, power use increases as the material removal rate does. The more fundamental problem, however, is that an increase of the production rate also implies an increase in material use.

A machine that works ten times faster also produces ten times more parts and needs as much more raw materials. And that's detrimental from an energy-saving perspective, because material production completely dominates industrial energy consumption. For example, the manufacture of a one kilogram steel part takes approximately 3 MJ for milling and 27 MJ for steel production. [11}

Lowering the specific energy consumption of CNC milling machines by increasing the production rate thus leads to an important increase in total energy use. This effect would not exist if one CNC machine replaced four manually operated milling machines and total production would remain at the same level. But that's not the case. CNC machine tools are used precisely because they can produce higher quantity of components in a shorter time and at lower costs. [13]

For a given operation, the machining time was reduced from 100 minutes at the beginning of the twentieth century to one minute using modern CNC machine tools. [23] There is still considerable scope for higher production rates: according to a forecast, the future upper limit for the cutting speed of a milling machine, based on the progress in cutting tool materials, will approach one Mach, or 340 metres per second. [23]

Non-Conventional Machining Technologies

The rise of CNC machine tools is not the only high-energy trend in manufacturing technology. At least as important is the emergence of so-called "non-conventional" machine tools. Conventional machining processes (such as milling and turning) remove material by applying forces on the material with a cutting tool that is harder than the material. Such forces induce plastic deformation within the workpiece leading to shear deformation and chip formation. [24]

Originally, non-conventional machine tools were developed specifically for workpiece materials that are difficult or impossible to shape using traditional processes. These can be stronger materials of high strength, or materials with high abrasive wear. In the first case, conventional machining is inadequate because it's time-consuming and leads to unacceptable tool wear. In the second case, conventional machining falls short because the tool bit can cause surface cracks and residual stresses in brittle materials. [26]

Laser Cutters

However, some non-conventional machine tools have found extensive use in machining of common materials, because they offer added benefits to traditional machine tools. [26] This is especially true for the laser cutter, which uses a highly concentrated energy beam. Contrary to milling, a three-dimensional process, laser cutters operate in only two dimensions. They have become popular for cutting common sheet metal, at the expense of more traditional technologies such as punching, blanking, and guillotining. [11] Metal cutting machines are the second most important machine tools in the manufacturing industry, after milling machines.

Compared to conventional machine tools, laser cutters offer a higher degree of precision and quality of finish. They can create more complex profiles and finer details. Cutting and drilling can be achieved with no force applied to the workpiece, avoiding damage. There is only a small heat-affected zone, which reduces the chances of warping the material (a problem with machine tools based on thermal energy). [26] [27] [4]

Unfortunately, laser cutters have one big disadvantage: they need a lot of power. Being CNC machines, they require substantial power for computer control and auxiliary functions. Apart from that, they also make use of a more energy-instensive cutting "tool" than the machines they replace. A laser cutter works by melting or evaporating material, which is energy-intensive compared to shearing or punching.

This means that laser cutters use even more power than CNC machines. Research has shown that a CO2 laser cutter with a 4 kW laser beam output requires 55 kW of power cutting steel plates at maximum output power, while a modern CNC shearing machine (a "guillotine" cutter) with a similar cutting force requires less than 16 kW of power at maximum output. [18][28]

Nobody has bothered to measure the power use of a hand-controlled (electrically powered) shearing machine, but looking at power requirements of similar machine tools, this must be less than 5 kW. [10] Even more efficient is a hand-powered shearing machine -- still for sale -- which requires no external power at all. As with 3D milling machines, we see an evolution in 2D metal cutting towards increasingly power-intensive manufacturing processes, and the increase in power use accelerates over time.

Punching Machines

When used for cutting sheet metal, laser cutters are most often used for making complex shapes instead of straight cuts. This makes them an alternative to punching machines, rather than to shearing or guillotining machines. Just like a laser cutter, a punching machine can cut free forms in 2D sheet metal through the process of nibbling, but compared to laser cutters they have lower resolution and produce parts with a rougher finish.

Nibbling requires much less power than laser cutting. To my knowledge, the power use of punching machines hasn't been researched, but the maximum power requirements indicated by manufacturers are between 6 and 11 kW. [29] A CNC punching machine thus requires at least five to ten times less power than a laser cutter.

I also found power requirements for several manually operated nibbling machines in a 1980s catalogue, which are between 1.5 kW (without digital readout) and 2.2 kW (with digital readout). [30] If we would produce a similar part on each of these machines, using a manual machine could thus be 25 to 35 times more efficient than using a laser cutter.

Higher power consumption can result in lower energy use per part if the production rate is high enough, but there seems to be no comparison of the energy use of laser cutters and CNC punching or shearing machines available. Figuring out the production rate of these machines is complicated, because it depends on many factors: material properties, material thickness, design complexity, positioning system, and so on.

Nevertheless, from the scattered data I could gather, it seems unlikely that laser cutters are fast enough to compensate for their higher power requirements for most metal cutting jobs. CNC shearing machines are faster than laser cutters for cutting straight lines, and CNC punching machines are faster than laser cutters for many common metal-cutting jobs. The unique selling point of the laser cutter lies less in speed and more in accuracy, product quality, and flexibility.

A crucial advantage is that a laser cutter needs just one tool, no matter how complex the design of the workpiece. On a CNC punching machine you need to load the right selection of tools for each specific design, and possibly introduce tool changes along the way. This means that the more complicated the design is, the faster and more advantageous a laser cutter becomes compared to a punching machine. This is even higher in small production runs, because there is no need to set up tools.

Laser cutters are thus especially suited for complex quality parts in small batches. This market will grow because as laser cutters spread they will raise the standards of production. Precisely because of the availability of laser cutters, products will get more complex, more accurate, more qualitative, and more flexible.

Fibre Laser Cutters

For laser cutters, energy costs account for about 25% of total production costs. [10] This means that there is more impetus to develop energy-efficient technology, and addressing the power use of the laser source is most profitable. While the CO2-laser is the most commonly used technology in sheet metal industry, the more efficient fibre laser source technology is an emerging alternative. Measurements indicate that a 2 kW fibre laser draws 17.6 kW at maximum laser output, which compares to 33 kW for a 2.5 kW CO2-laser and 59 kW for a 4 kW CO2-laser. [18]

The maximum power requirements of fibre lasers are thus about two to three times lower than those of a CO2-laser. This comes close to the maximum power use of a CNC shearing machine (16 kW), but it is still double that of a CNC punching machine, and about 10 times higher than the power output of a hand-controlled (electrically powered) nibbling machine.

Furthermore, fibre laser cutters cannot replace CO2-laser cutters for all applications, at least not yet. The technologies complement each other. For sheet metal thicker than 4 to 6 mm, CO2 lasers are still the best choice regarding energy efficiency, working speed and cutting quality. [10][20][18] Fibre lasers cannot cut wood or acrylics, but they can cut brass and copper. For CO2-lasers, it's the other way around.

Manufacturing Becomes Increasingly Energy Intensive

By looking at two of the most important machining processes -- metal milling and cutting -- it becomes clear that our manufacturing methods are becoming more energy-intensive. This is remarkable, because there are many reasons to believe that we should move in the opposite direction. Our dependence on non-renewable energy sources has created tough challenges, like climate change, peak oil, safekeeping nuclear waste, environmental degradation, and geopolitical conflicts. If we plan a switch to a manufacturing industry based on renewable energy, we would be smart to evolve to less energy-intensive machine tools instead.

Automated machine tools can never become as energy efficient as their hand-controlled counterparts. Replacing human operators requires energy. A fully automated machine will always consume more energy than a semi-automated machine. So, choosing fewer automated manufacturing technologies should be at least part of the solution.

A manually operated turning machine. Picture: Wikipedia Commons.

Energy use is not the only concern. CO2PE, an initiative that aims to coordinate international efforts to document, analyse and improve the environmental footprint of manufacturing processes, notes that:

"The intensifying use of non-conventional processing techniques, such as electro-chemical and laser based processes, results in the generation of emissions that have hardly been investigated from an environmental perspective. These undocumented and hard to control material flows are likely to imply significant potential human health hazards." [31]

CO2PE and similar research efforts do not aim to promote fewer automated machine tools. Instead, they aim to promote making automated machine tools as efficient (and safe) as possible. This strategy will spur economic growth, but won't prevent the increase of energy consumption in manufacturing. While energy efficiency improvements will be realized, Dahmus and Gutowski note that "efficiency improvements in auxiliary equipment may in fact lead to increases in the sales of auxiliary equipment." [13]

Manufacturing technology is a posterchild for the paradox of energy-efficiency. [32] The more efficient digital machines become, the more they replace older generations of machine tools and find new applications, and the larger their influence in total energy use will be. Likewise, improvements in energy-efficiency pave the way for ever larger and more powerful machines, as machine tools keep growing in size, speed and complexity. [33]

The Maker Revolution

Some CNC machine tools are now available in desktop sizes, with prices that bring them within reach of richer western customers. For example, you can now find a desktop CNC milling machine for less than € 5,000, and a desktop laser cutter for about € 12,000. Digital machine tools and non-conventional machine tools also appear in fab labs, makerspaces, and hackerspaces, where they are accessible to members.

There is quite some difference between desktop CNC machines and the CNC machines used in industry. A desktop CNC milling machine can only mill wood or aluminium, and a desktop laser cutter only cuts thin wood, plastics or cardboard. However, some fab labs and makerspaces have been introducing industry-scale machine tools lately, and the general expectation is that the possibilities of desktop manufacturing tools will improve quickly. Furthermore, there isn't a strict separation between the digital revolution inside and outside factories. Consumers can already send digital designs to "cloud manufacturers", which use industrial-scale digital machine tools to produce whatever you can design on your computer. [34]

A manually operated vertical milling machine. It has a control panel and a digital readout, but the operator is in control of all movements. Picture: lathes.co.uk

Some environmentalists have embraced digital machine tools. However, this consumer-driven digital maker revolution is just as unsustainable as the digital fabrication revolution in large factories. We've been able to produce everything locally for decades using hand-controlled machine tools. Digital tools only allow us to produce more and faster. And this happens, just as in industry, at the expense of higher energy consumption.

It's not only about the higher energy use of the machines. If consumers can use fast manufacturing machines, total material production will likely increase, similar to what is happening in factories. In his 2012 book Makers: The New Industrial Revolution, Chris Anderson writes that:

"The digital transformation of making stuff is doing more than simply making existing manufacturing more efficient. It's also extending manufacturing to a hugely expanded population of producers -- the existing manufactures plus a lot of regular folk who are becoming entrepreneurs. We will all just be a click away from getting factories to work for us. What do you want to make today?" [34]

When billions of people are just a click away from getting factories to work for them, whether in the cloud or on their desktops, this does not bode well for sustainability. We'll create even more stuff, and each product will cost much more energy than if produced with conventional methods.

[14] Other studies have confirmed these results for other types of CNC machine tools. For instance, a comparison of the specific energy consumption of six different grinding processes revealed that the energy required for the grinding process itself is just 10-20% of the energy of the entire grinding machine tool. [see 18, page 37]

[15] While the manual milling machine cuts steel at a rate of 0.35 cubic centimetres per second, the CNC milling machines from 1988 and 1998 achieve a material removal rate of 1.2 cubic centimetres per second, and the machine from 2000 obtains a rate of 4.7 cubic centimetres per second. Cited from note 13.

[33] This is very similar to what has happened with cars, where more efficient engines brought us more efficient small cars but also very power hungry SUV's, which would never have gotten so popular were it not for energy-efficient engines. Contrary to cars, which cannot get infinitely larger and faster because they have to adapt to road sizes and traffic laws, there is no upper limit when it comes to building larger and faster machine tools. Maybe all we need is a maximum speed limit for machine tools.

Kris De Decker is the creator and author of "Low-tech Magazine", a blog that is published in English, Dutch and Spanish. Low-tech Magazine refuses to assume that every problem has a high-tech solution. (Since 2007).

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